Radar apparatus

ABSTRACT

A radar apparatus is provided which includes a plurality of antennas arranged at unequally spaced intervals in a single row, the plurality of antennas configured to transmit a radio wave as transmitting antennas during continued multiple periods, and receive a reflected radio wave as receiving antennas during the continued multiple periods. A plurality of transceivers are included for transmitting a radio wave from the transmitting antennas and receiving received signals from the receiving antennas, said received signals representing the reflected wave received at the receiving antennas. A signal processing unit is included for selecting a transceiver which transmits a radio wave during each of the continue multiple periods, selecting transceivers which receive the received signals from the receiving antennas during each of the continued multiple periods, and perform digital beam forming with a first receiving signal channel group and a second receiving signal channel group.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority of Japanese Patent ApplicationNumber 2004-182537, filed on Jun. 21, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radar apparatus that uses afrequency-modulated continuous wave (FW-CW) as a transmit wave and, moreparticularly, to a radar apparatus that performs scanning for receivingreflected waves of a transmitted radiowave by digital beam forming(DBF).

2. Description of the Related Art

Various radar apparatuses, of a type that performs scanning on receiveby using DBF, have been developed and are known in the prior art.Generally, the basic configuration of this type of radar apparatusemploys a single transmitting antenna and a plurality of receivingantennas, and a radiowave is transmitted from the transmitting antennaand reflected waves of the transmitted radiowave are received by theplurality of antennas.

However, the radar apparatus of this configuration requires as manyreceivers as there are receiving antennas, and a large number ofreceivers must be provided if the scanning accuracy is to be enhanced.This has presented the problem that as the number of receiversincreases, the weight and size of the apparatus increases and, what ismore, a large amount of power is needed.

To solve the above problem, radar apparatuses that achieve size andweight reductions are proposed, for example, in Japanese UnexaminedPatent Publication Nos. S63-256879, H11-311668, and H11-160423. Suchradar apparatuses are constructed so that a plurality of receivingantennas are connected to a single receiver via a switch. Alternatively,a plurality of receiving antennas are divided into several groups, thatis, a receiving antenna array comprising a large number of receivingantennas is divided into subarrays each with four receiving antennas,for example, and one receiver is provided for each subarray of fourreceiving antennas, the four receiving antennas being connected to thereceiver via a switch. When receiving reflected waves of a transmittedradiowave, the plurality of antennas are sequentially switched forconnection to the receiver. In this way, radar signals received at therespective receiving antennas can be obtained in a time-divisionfashion.

With this configuration, the number of receivers can be reduced to oneor to less than the number of receiving antennas, thus achieving areduction in the size and cost of the apparatus.

Here, the radiowave used by the radar apparatus is a radiowave in ahigh-frequency band such as a 76-GHz band. Accordingly, the signalshandled in the transmission paths from the receiving antennas to thereceiver are also high-frequency band signals. The number of inputs on aswitch that can switch such high-frequency signals is generally two orthree.

Accordingly, when switching between four or more receiving antennas, itis practiced to use a plurality of switches. For example, unit switches,each being a single-pole double-throw (SPDT) or a single-polethree-throw (SP3T) switch, are combined in tree type to achieve multipleswitching. Here, a high-frequency switch of planar circuit type, such asa monolithic microwave integrated circuit (MMIC) or a hybrid integratedcircuit (HIC), is used as the unit switch.

However, when the switches are connected in multiple stages, signalattenuation increases as the signal passes through each switch, andhence the problem that, as the number of stages of switches is increasedto reduce the number of receivers, reception sensitivity decreases,appears.

In view of this, a radar apparatus relatively simple in configurationand capable of preventing the degradation of reception sensitivity isproposed, for example, in Japanese Unexamined Patent Publication No.2000-155171. While the radar apparatuses earlier described comprise asingle transmitting antenna and a plurality of receiving antennas, theradar apparatus disclosed in this patent document employs a plurality oftransmitting antennas, which are used by being switched from one toanother, and thereby reduces the number of receiving antennas as well asthe number of switches used to switch between the receiving antennas. Itis claimed that with this configuration, reception sensitivity can beincreased, while reducing the cost of the apparatus by reducing thenumber of antennas and switches.

This radar apparatus comprises four transmitting antennas A1 to A3 andtwo receiving antennas A4 and A5 arranged in a straight line on the sameplane, the total number of antennas being smaller than that in any ofthe prior known radar apparatuses. It is claimed that this radarapparatus configuration makes the fabrication of the radar apparatuseasier, can reduce the cost and, in applications such as automotiveradar, can make the overall shape of the radar suitable for mounting ona vehicle.

According to the above radar apparatus, the attenuation of receivedsignals due to switches can be reduced, and a number of channels largerthan the number of antennas can be achieved with fewer antennas.According to the above radar apparatus, more channels can be achieved inDBF than there are antennas and, to obtain a narrower beam of higherdirectivity and, for example, to achieve nine channels, the number ofantennas can be reduced to six, compared with 10 required in the priorart configuration, but when the antennas are mounted to construct theradar apparatus, as the antennas are arrayed in a single row, the sixantennas require a mounting space equivalent to ten antennas.

On the other hand, when such a radar apparatus is mounted as anelectronic apparatus, for example, on an automobile or the like, theposition where it can be mounted so as to transmit the radiowave forwardis limited, and the available mounting space is quite restricted. Theradar apparatus used in such an environment must be made as small aspossible. Accordingly, while the number of antennas can be reduced, theabove radar apparatus still leaves much to be desired when it comes tosize reduction. Furthermore, from the standpoint of vehicle drivingsafety, it is desired to further increase the performance of the radarapparatus for recognizing targets ahead of the vehicle, and it is alsodesired to reduce the cost of the apparatus.

Accordingly, it is an object of the present invention to provide a radarapparatus that can achieve a size reduction, a performance increase, anda cost reduction while achieving multiple channels, by minimizing thenumber of antennas necessary to perform scanning by digital beam forming(DBF) for reception of the reflected waves of a transmitted radiowave,and that can also achieve enhancements in the speed and accuracy ofazimuth detection.

SUMMARY OF THE INVENTION

To solve the above problem, according to the present invention, there isprovided a radar apparatus which comprises a plurality of antennashaving identical antenna characteristics and arranged at unequallyspaced intervals in a single row, a transmitter for transmitting aradiowave from at least one antenna selected from among the plurality ofantennas, a receiver for receiving a reflected wave of the transmittedradiowave at each of the antennas, and a signal processing unit forperforming digital beam forming based on a received signal representingthe received reflected wave, the radar apparatus further comprising: afirst selector switch for supplying a transmit signal of the radiowaveto each of the antennas by sequentially selecting the antennas; and asecond selector switch for supplying the received signal, representingthe reflected wave received at each of the antennas, to the receiver bysequentially switching the antennas for connection to the receiver, andwherein: in accordance with a receiving signal channel switchingsequence relating to antenna transmission and reception for the digitalbeam forming, when the antennas are sequentially selected by the firstselector switch and the radiowave is transmitted from the selectedantenna for each cycle of the transmit signal, the second selectorswitch selects from among the plurality of antennas an antenna forreceiving the reflected wave of the transmitted radiowave and suppliesthe received signal to the receiver. Here, all of the antennas can beused for both transmission and reception.

When the receiving signal channel switching sequence selects onlyodd-numbered receiving signal channels or only even-numbered receivingchannels, the second selector switch selects the signal received via theantenna if the received signal corresponds to any one of theodd-numbered receiving signal channels or the even-numbered receivingchannels, or when the receiving signal channel switching sequenceselects only the odd-numbered receiving signal channels or only theeven-numbered receiving channels, the receiver supplies the receivedsignal to the signal processing means if the received signal correspondsto any one of the odd-numbered receiving signal channels or theeven-numbered receiving channels.

The receiving signal channel switching sequence includes a switchingsequence for selecting all receiving signal channels and a switchingsequence for selecting only the odd-numbered receiving signal channelsor only the even-numbered receiving channels, and either one of theswitching sequences is selected in accordance with an environment inwhich the signal processing unit performs azimuth detection.

Alternatively, when the receiving signal channel switching sequenceselects only receiving signal channels located in a left half portion,the second selector switch selects the signal received via the antennaif the received signal corresponds any one of the receiving signalchannels in the left half portion, and when the receiving signal channelswitching sequence selects only receiving signal channels located in aright half portion, the second selector switch selects the signalreceived via the antenna if the received signal corresponds any one ofthe receiving signal channels in the right half portion.

Further, when the receiving signal channel switching sequence selectsonly the receiving signal channels located in the left half portion, thereceiver supplies the received signal to the signal processing unit ifthe received signal corresponds any one of the receiving signal channelsin the left half portion, and when the receiving signal channelswitching sequence selects only the receiving signal channels located inthe right half portion, the receiver supplies the received signal to thesignal processing unit if the received signal corresponds any one of thereceiving signal channels in the right half portion; here, the signalprocessing unit performs processing for azimuth detection for thereceiving signal channels in the left half portion and for the receivingsignal channels in the right half portion, separately.

In the radar apparatus of the present invention, the plurality ofantennas are arranged such that the ratio of the antenna spacing betweena predetermined pair of adjacent antennas to the antenna spacing betweenanother pair of adjacent antennas is 1:2, wherein the plurality ofantennas include first to fourth antennas arrayed in sequence along astraight line, and the first and second antennas are arranged with afirst spacing while the second and third antennas and the third andfourth antennas, respectively, are arranged with a second spacing, thesecond spacing being twice as large as the first spacing. Further, thesignal processing unit forms multiple digital beams of eleven channelsbased on the received signals received at the first to fourth antennas.

The first antenna sequentially transmits radiowaves over a plurality ofcycles of the transmit signal, and reflected waves of the radiowavessequentially transmitted cycle by cycle are sequentially received by thesecond antenna, the third antenna, and the fourth antennas in thisorder; then, the second antenna, the third antenna, and the fourthantennas in this order sequentially transmit the transmit signal withthe same cycle, the reflected wave of each transmitted radiowave isreceived by the first antenna, and the received signal that correspondsto any one of the odd-numbered receiving signal channels or theeven-numbered receiving channels is supplied to the signal processingunit.

The first antenna and the second antenna alternately transmit thetransmit signal with the same cycle, the third antenna and the fourthantenna receive reflected waves of the radiowaves sequentiallytransmitted with the same cycle, and received signals corresponding tothe receiving signal channels located in the left half portion of theplurality of receiving signal channels are supplied to the signalprocessing unit; then, the third antenna and the fourth antennaalternately transmit the transmit signal with the same cycle, the firstantenna and the second antenna receive reflected waves of the radiowavessequentially transmitted with the same cycle, and received signalscorresponding to the receiving signal channels located in the right halfportion of the plurality of receiving signal channels are supplied tothe signal processing unit.

In the radar apparatus of the present invention, each of the pluralityof antennas is provided with a transmitting port and a receiving port,and a transmitter is connected to each transmitting port, while areceiver is connected to each receiving port; here, each transmittingport is selectively connected to a common transmitter, and eachreceiving port is connected to a common receiver.

Each of the plurality of antennas is provided with a bidirectionalswitch for switching between transmission and reception, and thebidirectional switch, when thrown to a position for transmission,connects an associated one of the antennas to the transmitter and, whenthrown to a position for reception, connects the associated one of theantennas to the receiver.

A transceiver in which an output port of the transmitter and a receivingport of the receiver are shared for transmission and reception isprovided for each of the plurality of antennas, wherein the transceiveris provided common to the plurality of antennas and is connected to aselected one of the antennas for transmission or reception; here, thenumber of transceivers provided is smaller than the number of theplurality of antennas, and the transceiver is connected by atime-division switch to the selected antenna for the transmission or thereception.

The radar apparatus of the present invention further comprises avoltage-controlled oscillator for supplying a reference signal to thetransmitter and the receiver, wherein the voltage-controlled oscillatoris shared by the transmitter and the receiver that are provided commonto the plurality of antennas.

In the radar apparatus of the present invention, the plurality ofantennas include four antennas arranged at unequally spaced intervalsalong a straight line, and the signal processing unit is configured tobe able to form multiple digital beams of eleven channels based on thereceived signals received at the respective antennas in accordance withthe receiving signal channel switching sequence, or the signalprocessing unit is configured to be able to form multiple digital beamsof a plurality of channels fewer than eleven channels based on some ofthe received signals received at the respective antennas in accordancewith the receiving signal channel switching sequence.

As described above, the radar apparatus of the present invention isconstructed to transmit a radiowave from at least one antenna selectedfrom among the plurality of antennas arranged in a straight line on thesame plane, and to receive the reflected waves of the transmittedradiowave at the respective antennas; accordingly, not only can a largernumber of channels be obtained with fewer antennas than that inconventional radar apparatuses, but also the size and cost of the radarapparatus can be reduced. Further, when all of the plurality of antennasare used for both transmission and reception, the number of channels canbe greatly increased, and the directivity when the received signals arecombined can be increased, serving to enhance the performance of theradar apparatus.

Further, in the radar apparatus of the present invention, as the numberof antennas is reduced compared with the prior art radar apparatusconfiguration, and as the plurality of antennas are arranged at astrategically chosen spacing, digital beams of multiple channels can beformed, while achieving reductions in the size and the cost of theapparatus.

In the radar apparatus of the present invention, as the formation ofdigital beams of multiple channels is achieved with a minimum number ofantennas by arranging the plurality of antennas at a strategicallychosen spacing, the fine channel switching sequence which uses all thechannels or the coarse channel switching sequence which uses only theodd-numbered channels can be selected according to the using environmentof the radar apparatus; in normal operation, the coarse channelswitching sequence is selected to increase the processing speed forazimuth detection.

Furthermore, in the radar apparatus of the present invention, as theformation of digital beams of multiple channels is achieved with aminimum number of antennas by arranging the plurality of antennas atstrategically chosen spacing, all the channels can be divided into aleft-half group and a right-half group to provide a left-side channelswitching sequence and a right-side channel switching sequence so thatthe processing for azimuth detection can be performed twice for the sametarget object; this serves to enhance the accuracy of azimuth detection.

The radar apparatus configuration described above makes the fabricationof the radar apparatus easier and achieves a reduction in cost and, whenapplying the radar apparatus, for example, as an anti-collision radarapparatus to be mounted on a vehicle or the like, the overall shape ofthe radar apparatus can be made suitable for mounting on the vehicle,and the processing for azimuth detection can be made faster andefficient, offering advantages in collision avoidance applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, objects and advantages of the present invention willbecome apparent from the following description of preferred embodimentswith reference to the drawings in which like reference charactersdesignate like or corresponding parts throughout several views, and inwhich:

FIG. 1 is a schematic block diagram showing the configuration of a radarapparatus according to the present invention;

FIG. 2 is a diagram for explaining the operating principle of antennasin the radar apparatus according to the present invention;

FIG. 3 is a diagram for explaining the receiving operations of theantennas in the radar apparatus according to the present invention;

FIG. 4 is a time chart diagram for explaining a first specific exampleof antenna switching operation in the radar apparatus according to thepresent invention;

FIG. 5 is a diagram for explaining the formation of receiving channelsin the first specific example of antenna switching operation;

FIG. 6 is a time chart diagram for explaining a second specific exampleof antenna switching operation in the radar apparatus according to thepresent invention;

FIG. 7 is a diagram for explaining the formation of receiving channelsin the second specific example of antenna switching operation;

FIG. 8 is a time chart diagram for explaining a first embodiment ofantenna switching operation in the radar apparatus according to thepresent invention;

FIG. 9 is a diagram for explaining the formation of receiving channelsin the first embodiment of antenna switching operation;

FIG. 10 is a time chart diagram for explaining a second embodiment ofantenna switching operation in the radar apparatus according to thepresent invention;

FIG. 11 is a diagram for explaining the formation of receiving channelsin the second embodiment of antenna switching operation;

FIG. 12 is a diagram for explaining a first modified example of antennatransmission/reception configuration in the radar apparatus according tothe present invention;

FIG. 13 is a diagram for explaining a second modified example of antennatransmission/reception configuration in the radar apparatus according tothe present invention;

FIG. 14 is a diagram for explaining a first specific example of antennaswitching means in the radar apparatus according to the presentinvention;

FIG. 15 is a diagram for explaining a second specific example of antennaswitching means in the radar apparatus according to the presentinvention;

FIG. 16 is a diagram for explaining a third specific example of antennaswitching means in the radar apparatus according to the presentinvention;

FIG. 17 is a diagram for explaining a fourth specific example of antennaswitching means in the radar apparatus according to the presentinvention;

FIG. 18 is a diagram for explaining a fifth specific example of antennaswitching means in the radar apparatus according to the presentinvention;

FIG. 19 is a schematic block diagram showing the configuration of aradar apparatus according to a previously proposed technique; and

FIG. 20 is a diagram for explaining the formation of receiving channelsin the receiving operations of antennas according to the radar apparatusshown in FIG. 19.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a better understanding of the effect to be achieved by the presentinvention, first the configuration of a previously proposed radarapparatus, that forms the basis for the present invention, will bedescribed in detail below. One example of the proposed radar apparatuswill be described with reference to FIGS. 19 and 20.

FIG. 19 is a schematic block diagram showing the configuration of theradar apparatus. In this radar apparatus, three transmitting antennasA1, A2, and A3 and receiving antennas A4 and A5 are connected to aswitching means 5. A transmitter 2, which includes an oscillator 3 suchas a voltage-controlled oscillator (VCO) that outputs a high-frequencysignal, for example, in a 76-GHz band, and a receiver 4 are connected tothe switching means 5. The receiver 4 sends the signal received from thereceiving antenna to a signal processing control unit 1 in synchronismwith the oscillator signal output from the oscillator 3. The signalprocessing control unit 1 performs signal processing for digital beamforming (DBF) based on the received signal, and also performs switchingcontrol for the transmitting antennas and receiving antennas via theswitching means 5.

The oscillator 3 is connected via a distributor in the transmitter 2 toa switch SW1 on the transmitting side via which the oscillator output issupplied to the transmitting antennas. This switch SW1 is a single-polethree-throw (SP3T) switch whose outputs are connected to the threetransmitting antennas A1, A2, and A3, respectively. By operating theswitch SW1 under instruction from the signal processing control unit 1,the high-frequency signal from the oscillator 3 is supplied to thetransmitting antennas A1, A2, and A3 in time division fashion. Thus, thehigh-frequency signal from the oscillator 3 is transmitted out from thetransmitting antennas A1, A2, and A3, in sequence, in a time-divisionfashion. The transmitting antennas A1, A2, and A3 used here have thesame directivity, i.e., the directivity that can radiate a radiowaveover the entire detection region.

On the other hand, the two receiving antennas A4 and A5 are provided onthe receiving side. A switch SW2 on the receiving side is connected tothe receiving antennas A4 and A5. This switch SW2 is a single-poledouble-throw (SPDT) switch whose outputs are connected to the receivingantennas A4 and A5. The single input of the switch SW2 is connected to amixer in the receiver 4. By operating the switch SW2 under instructionfrom the signal processing control unit 1, the received signals obtainedby the two receiving antennas A4 and A5 are selectively supplied to thereceiver 4.

FIG. 20 shows how switching is made between the antennas in the radarapparatus shown in FIG. 19. In this radar apparatus, when the spacingbetween the receiving antennas A4 and A5 is denoted by L, thetransmitting antennas A1, A2, and A3 are arranged and spaced apart by2L.

The radiowaves transmitted from the transmitting antennas A1, A2, and A3are reflected by a target object and received by the receiving antennasA4 and A5. Accordingly, when the transmitting antennas are moved inspace, if the receiving antennas are translated correspondingly in theopposite direction, the same received signals should be obtained.Therefore, when the radiowave is transmitted from the transmittingantenna A2, the received signals at the receiving antennas A4 and A5 arethe same as those received when the transmitting antenna A2 is moved tothe position of the transmitting antenna A1 and the receiving antennasA4 and A5 are translated in the opposite direction by a distance equalto the spacing L between the receiving antennas A4 and A5. Likewise,when the radiowave is transmitted from the transmitting antenna A3, thereceived signals at the receiving antennas A4 and A5 are the same asthose received when the transmitting antenna A3 is moved to the positionof the transmitting antenna A1 and the receiving antennas A4 and A5 aretranslated by 2L.

FIG. 20 shows the pairing relationship between the transmitting andreceiving antennas at each time instant and the positional relationshipin the direction in which the antennas are arranged. It can be seenthat, by appropriately operating the switches SW1 and SW2, beams of sixchannels can be obtained using a total of five antennas consisting ofthe three transmitting antennas and the two receiving antennas. This isequivalent to providing six receiving antennas for one transmittingantenna.

In another previously proposed radar apparatus, if six receivingantennas are to be provided for connection to one receiver, a two-stageswitch configuration comprising two single-pole double-throw switchesand one single-pole three-throw switch has had to be employed; bycontrast, in the radar apparatus shown in FIG. 19, only a single-stageswitch SW2 need be provided for switching between the receiving antennasA4 and A5, though the switch SW1 must be provided on the transmittingside.

Further, in the radar apparatus shown in FIG. 19, if it is desired toincrease the number of channels in order to increase the directivity ofthe obtained beams, an additional receiving antenna A6 (not shown)having the same antenna characteristics as the receiving antennas A4 andA5 is provided so as to be spaced L apart from the others. In this case,the spacing between the respective transmitting antennas A1, A2, and A3should be increased to 3L. According to a radar apparatus having such anantenna configuration, three channels are added to the channels shown inFIG. 20, and beams of nine channels can be obtained using a total of sixantennas.

It is claimed that the above radar apparatus configuration makes thefabrication of the radar apparatus easier, can reduce the cost and, inapplications such as automotive radar, can make the overall shape of theradar suitable for mounting on a vehicle. However, when such a radarapparatus is mounted, for example, on an automobile or the like, theposition where it can be mounted is limited, and the available mountingspace is quite restricted; therefore, the radar apparatus used in suchapplications must be made as small as possible. However, while thenumber of antennas can be reduced, the above radar apparatus stillleaves much to be desired when it comes to size reduction; furthermore,from the standpoint of vehicle driving safety, it is needed to furtherincrease the performance of the radar apparatus for recognizing targetsahead of the vehicle, and it is also desired to reduce the cost of theapparatus.

Next, embodiments of the radar apparatus that can achieve the earlierdescribed object of the present invention will be described withreference to the accompanying drawings. FIG. 1 shows in simplified formthe configuration of the radar apparatus according to the presentinvention. The radar apparatus shown here is basically the same as thepreviously proposed radar apparatus shown in FIG. 19 in that thescanning for receiving the reflected waves of the transmitted radiowaveis performed by signal processing of digital beam forming (DBF). In theradar apparatus shown in FIG. 1, the same parts as those of the radarapparatus shown in FIG. 19 are designated by the same referencenumerals. In the figure, the high-frequency circuit block within thedashed lines, which contains the transmitter 2, the voltage-controlledoscillator (VCO) 3, the receiver 4, and the switching means 5, isconstructed from a monolithic microwave integrated circuit (MMIC), as inthe prior art radar apparatus.

In the radar apparatus of FIG. 1, four antennas A1, A2, A3, and A4 forman antenna array A and are connected to the switching means 5. Thetransmitter 2, which includes the oscillator 3 such as avoltage-controlled oscillator (VCO) that outputs a high-frequencysignal, for example, in a 76-GHz band, and the receiver 4, whichreceives a reflected signal representing a reflection of the transmittedradiowave signal output from the transmitter, are connected to theswitching means 5. The receiver 4 is synchronized to the oscillatorsignal output from the oscillator 3, and sends the signal received fromthe receiving antenna to the signal processing control unit 1. Thesignal processing control unit 1 performs signal processing for digitalbeam forming (DBF) based on the received signal supplied from thereceiver 4, and also performs switching control for the four antennas A1to A4 via the switch SW in the switching means 5.

The major difference between the basic radar apparatus of FIG. 1 and theprior art radar apparatus is that, while, in the prior art radarapparatus, the transmitting antennas and the receiving antennas in theantenna array comprising the plurality of antennas are used only fortransmission or reception, respectively, the plurality of antennasforming the antenna array A in the radar apparatus shown in FIG. 1 arenot dedicated to transmission or reception but one or more or all of theplurality of antennas are used for both transmission and reception. Theplurality of antennas are suitably switched by means of the switch SWbetween the transmission of the transmit signal supplied from thetransmitter 2 and the reception of the reflected wave of the transmittedradiowave signal. The radiowave of the transmit signal is transmittedout from the sequentially selected antennas, and the reflected waves ofthe transmitted radiowave are received by the plurality of antennas,thus achieving scanning for multi-channel reception.

Further, the radar apparatus shown in FIG. 1 is characterized in thatthe plurality of antennas are arranged at unequally spaced intervals,rather than arranging them at equally spaced intervals as in the priorart radar apparatus. If the plurality of antennas were arranged atequally spaced intervals, the receiving position where each receivingantenna would receive the reflected wave of the radiowave transmittedfrom a selected one of the antennas would only be shifted by a distanceequal to the antenna spacing. This would lessen the advantage ofsequentially switching the transmitting antennas, and would make itdifficult to increase the number of channels. In view of this, a widerantenna spacing and a narrower antenna spacing are provided, the widerspacing being twice as large as the narrower spacing. With this antennaarrangement, a larger number of channels can be obtained with fewerantennas.

The wider antenna spacing is not limited to twice the narrower spacing,but what is important is that the receiving position where eachreceiving antenna receives the reflected wave is shifted by a distanceequal to the antenna spacing; by shifting a plurality of antennas, alarger number of channels can be obtained with fewer antennas. Forexample, the wider antenna spacing may be set at 1.5 times the narrowerspacing. The following description is given by taking as an example thecase in which the wider antenna spacing is set at twice the narrowerspacing.

It is preferable that the plurality of antennas used in the radarapparatus shown in FIG. 1 have the same antenna characteristics interms, for example, of directivity and gain. Further, each antennashould have such a directivity that it can radiate the radiowave overthe entire detection region. It is also preferable that the antennas arearranged in a single row so that their transmitting and receivingsurfaces are aligned in a straight line. If the antenna characteristicsvary among the antennas, the amount of computation performed to detectthe phases contained in the received signals will increase, which willaffect the performance of the radar apparatus.

The radar apparatus shown in FIG. 1 is an example of the apparatusequipped with four antennas. The basic concept of digital beam forming(DBF) in this radar apparatus will be described with reference to FIGS.2 and 3. DBF is performed in the signal processing control unit 1; here,each received signal from the array antenna array A comprising theplurality of antennas is A/D converted into a digital signal, and theadjustments of beam scanning and sidelobe characteristics, etc. areaccomplished by digital signal processing.

FIG. 2 shows how the four antennas A1, A2, A3, and A4 in the radarapparatus are arranged horizontally in a row. In the figure, eachantenna is represented by a triangle, and four antennas are arranged.When the spacing between the antennas A1 and A2 is denoted by d, thespacing between the antennas A2 and A3 and the spacing between theantennas A3 and A4 are each set at 2d which is twice the spacing dbetween the antennas A1 and A2. To make the spacing easier to view,dashed triangles are shown, one between the antennas A2 and A3 and theother between the antennas A3 and A4, to indicate that the respectiveantennas are spaced by a distance equal to one antenna, i.e., 2d.

First, suppose that a radiowave is transmitted as a transmitted signalT1 from the first selected antenna A1, as shown in FIG. 2. The shadedtriangle in the figure indicates the transmitting antenna selected fortransmitting the radiowave. The radiowave transmitted from thetransmitting antenna A1 is reflected by a target object, and itsreflected wave returns to the antenna array A. The radiowave arrivingfrom the direction of angle θ relative to the center direction of theradar is received by the antenna array comprising the four antennas A1,A2, A3, and A4 arranged as shown in the figure. Compared with thepropagation path length that the reflected wave R11 travels beforearriving at the antenna A1, the reflected wave R21 arriving at theantenna A2, the reflected wave R31 arriving at the antenna A3, and thereflected wave R41 arriving at the antenna A4 travel farther by p, 3p,and 5p, respectively (where p=d sin θ) as shown in the figure.

This means that the reflected waves R21, R31, and R41 arriving at theantennas A2, A3, and A4 are retarded by the respective amounts from thereflected wave R11 arriving at the antenna A1. The amounts ofretardation are (2πd·sin θ)/λ, (6πd·sin θ)/λ, and (10πd·sin θ)/λ,respectively, where λ is the wavelength of the reflected wave.

When the reflected waves R11, R21, R31, and R41 are received by therespective antennas A1, A2, A3, and A4, the resulting received signalsS11, S21, S31, and S41 are supplied to the receiver 4 via the switch SW.Since the arrival time of the reflected wave differs from one antenna toanother, the phases of the respective received signals S21, S31, and S41are retarded by (2πd·sin θ)/λ, (6πd·sin θ)/λ, and (10πd·sin θ)/λ,respectively, from the phase of the received signal S11.

If the phase of each received signal is advanced by an amount equal tothe amount of retardation of the received signal by the digitalprocessing performed in the signal processing control unit 1, then thesame result is obtained as when the reflected waves from the θ directionare received in phase by all the antennas, and the directivity of allthe antennas is thus aligned in the θ direction.

Next, when the transmitting antenna is switched from the antenna A1 tothe antenna A2, as shown in FIG. 3, a radiowave is transmitted as atransmitted signal T2 from the antenna A2. The radiowave transmittedfrom the transmitting antenna A2 is reflected by a target object, andits reflected wave returns to the antenna array A. As in FIG. 2, thereflected wave arriving from the direction of angle θ relative to thecenter direction of the radar is received by the antenna arraycomprising the four antennas A1, A2, A3, and A4.

The antennas A1, A2, A3, and A4 respectively receive the reflected wavesR12, R22, R32, and R42 corresponding to the transmitted signal T2. Whenthe reflected waves R12, R22, R32, and R42 are received by therespective antennas A1, A2, A3, and A4, the resulting received signalsS12, S22, S32, and S42 are supplied to the receiver 4 via the switch SW.Here, compared with the propagation path length that the reflected waveR12 travels before arriving at the antenna A1, the reflected wave R22arriving at the antenna A2, the reflected wave R32 arriving at theantenna A3, and the reflected wave R42 arriving at the antenna A4 travelfarther by p (=d sin θ), 3p, and 5p, respectively, as shown in thefigure.

However, unlike the case of FIG. 2, in the case of FIG. 3, as thetransmitting antenna is shifted from the antenna A1 to the antenna A2,the arrival time of the reflected wave at the antenna A2 is set as thereference. As a result, compared with the case of FIG. 2, the arrivingposition of the reflected wave at each antenna is shifted in thehorizontal direction by an amount equal to the amount of shift from theantenna A1 to the antenna A2.

Then, compared with the received signal S22 resulting from the reflectedwave R22 received at the antenna A2, the phase of the received signalS12 from the antenna A1 is advanced by (2πd·sin θ)/λ, while the phasesof the received signals S32 and S42 resulting from the reflected wavesR32 and R42 arriving at the respective antennas A3 and A4 are retardedfrom the phase of the received signal S22 received at the antenna A2.The amounts of retardation here are (4πd·sin θ)/λ and (8πd·sin θ)/λ,respectively.

Accordingly, in the digital processing of each antenna received signalin the signal processing control unit 1, the received signal S12 havinga phase lead is retarded in phase according to the amount of the phaselead. On the other hand, the received signals S32 and S42 each having aphase delay are advanced in phase according to the respective amounts ofphase delay; then, the same result is obtained as when the arrivingreflected waves are received in phase by all the antennas, and thedirectivity of all the antennas is thus aligned in the θ direction.

When an FM-CW wave is used as the transmitted signal, the FM-CW wave istransmitted by sequentially selecting the antennas A1, A2, A3, and A4 asthe transmitting antenna for each period comprising increasing anddecreasing triangular wave sections of the FM-CW wave by controlling theswitch SW of the radar apparatus as described above. In each period, thereflected waves of the transmitted signal are received by the antennasA1, A2, A3, and A4. In this way, by transmitting the radiowave from aselected one of the four transmitting/receiving antennas and byreceiving the reflected waves of the transmitted radiowave by the fourantennas, eleven channels can be achieved with a space equivalent to sixantennas.

FIGS. 4 and 5 show how the eleven receiving channels are achieved with aspace equivalent to six antennas in the antenna array using the fourantennas. FIG. 4 shows a first specific example of the antenna switchingoperation in the radar apparatus. The timing for switching between thetransmitting antennas A1 to A4 and the receiving antennas A1 to A4 isshown in time series in conjunction with the triangular wave cycles ofthe transmitted FM-CW wave. In FIG. 4, receiving antenna 1 indicateseach of the transmitting antennas A1 to A4 that was selected fortransmission of the FM-CW wave and that, upon transmission, was switchedto a receiving antennas for reception of the reflected wave. On theother hand, antenna 2 indicates each of the receiving antennas A1 to A4that was switched to function as a receiving antenna other than thereceiving antenna 1.

FIG. 5 shows how the receiving channels are formed, that is, thereceiving signal channel switching sequence relating to antennatransmission and reception, in the first specific example of the antennaswitching operation performed in accordance with the switching timingchart shown in FIG. 4. In FIG. 5, the horizontal axis represents thetime, and triangles indicate receiving antennas, while shaded trianglesindicate antennas used for both transmission and reception. Thereceiving channels for the respective antennas are shown by reference tothe transmitting antenna for each period of the triangular FM-CW wave.In the illustrated example, each period consists of three triangularwaves in the FM-CW wave shown in FIG. 4. Accordingly, the first tofourth periods together contain 12 triangular waves.

The spacing between the antennas A2 and A3 and the spacing between theantennas A3 and A4 are each set at twice the spacing between theantennas A1 and A2. Therefore, in the first period of the FM-CW wave,the antenna A1 receives the reflected wave on channel 6, the antenna A2on channel 7, the antenna A3 on channel 9, and the antenna A4 on channel11.

In the second period of the FM-CW wave, as the transmitting antenna isswitched to the antenna A2, and the phase of the received signal at theantenna A2 is used as the reference, the arriving positions of thereflected waves at the respective antennas are apparently shifted fromthose in the first period by a distance equal to the spacing d. As aresult, in the second period, the antenna A1 receives the reflected waveon channel 5, the antenna A2 on channel 6, the antenna A3 on channel 8,and the antenna A4 on channel 10.

In the next third period, the transmitting antenna is switch to theantenna A3, and in the fourth period, it is switched to the antenna A4.Therefore, in each of these periods, the arriving positions of thereflected waves at the respective antennas are apparently shifted fromthose in the preceding period by a distance equal to the spacing 2d.Accordingly, in the third and fourth periods, similarly to the first andsecond periods, the reflected waves are received at the respectiveantennas at the positions shifted by the spacing 2d. As a result, whenviewed by reference to the antenna that transmitted the radiowave, thismeans that in the one switching cycle from the first period to thefourth period, the reflected waves of the transmitted wave have beenreceived on all the channels from channel 1 to channel 11.

Here, duplicated reception occurs on the sixth channel, but thisduplication is unavoidable because, in the case of FIG. 5, this channelis set as the reference channel. As for the duplicated reception on eachof the fourth and eighth channels, on the other hand, either one of thereceived signals is not necessary. Here, if the computation burden isconsidered, provision may be made to not receive the redundant signal bycontrolling the switch SW, or to receive the redundant signal but notperform signal processing on the received signal.

The antenna switching operation that implements the above provision isshown as a second specific example in FIGS. 6 and 7. In FIG. 6, as inFIG. 4, the timing for switching between the transmitting antennas A1 toA4 and the receiving antennas A1 to A4 is shown in time series inconjunction with the triangular wave cycles of the transmitted FM-CWwave. In FIG. 6, receiving antenna 1 indicates each of the transmittingantennas A1 to A4 that was selected for transmission of the FM-CW waveand that, upon transmission, was switched to a receiving antennas forreception of the reflected wave. On the other hand, antenna 2 indicateseach of the receiving antennas A1 to A4 that was switched to function asa receiving antenna other than the receiving antenna 1.

FIG. 7 shows how the receiving channels are formed, that is, thereceiving signal channel switching sequence relating to antennatransmission and reception, in the second specific example of theantenna switching operation performed in accordance with the switchingtiming chart shown in FIG. 6. In FIG. 7, as in FIG. 5, the horizontalaxis represents the time, and triangles indicate receiving antennas,while shaded triangles indicate antennas used for both transmission andreception. The receiving channels for the respective antennas are shownby reference to the transmitting antenna for each period of thetriangular FM-CW wave.

The receiving signal channel switching sequence relating to antennatransmission and reception in the second specific example differs fromthat in the first example in that, in the timing chart of FIG. 6, theantenna A3 is selected as the transmitting antenna only in onetriangular wave cycle of the FM-CW wave and, in that cycle, only theantenna A1 is switched to function as the receiving antenna 2. On theother hand, as shown by dashed triangles in FIG. 7, neither the antennaA2 nor the antenna 4 is selected as the receiving antenna 2 in the thirdperiod.

This is because the antenna A3 has been selected as the receivingantenna 2 for the eighth channel in the second section, and also becausethe antenna 3 is scheduled to be selected for the fourth channel in thefourth section. In this way, duplication is avoided on the same channel.With this receiving signal channel switching sequence, eleven receivingchannels are achieved with a space equivalent to six antennas in theradar apparatus equipped with the unequally spaced four antennas A1 toA4 as shown in FIG. 1. In the one switching cycle from the first sectionto the fourth section, multiple beams can be formed with ten triangularwaves in the FM-CW wave, and thus the signal processing speed can beenhanced.

In the first and second specific examples, the receiving signal channelswitching sequence has been described that achieves eleven channels witha space equivalent to six antennas in the radar apparatus equipped withthe unequally spaced four antennas A1 to A4 as shown in FIG. 1;according to this receiving signal channel switching sequence, elevenreceiving signal channels are always obtained in the operation of theradar apparatus, achieving reductions in the size and cost of the radarapparatus, and the directivity, when the received signals are combined,serving to enhance the performance of the radar apparatus.

Such a radar apparatus may be mounted, for example, on a vehicle andused to detect the azimuth to a target object located ahead. In suchapplications, there are cases where high azimuth detection performanceis needed, and cases where the speed of azimuth detection is morecritical than the performance itself. According to the radar apparatusshown in FIG. 1, eleven channels can be achieved with a space equivalentto six antennas, contributing to the reduction of the size and cost, butif eleven receiving channels are always obtained, the speed of azimuthdetection required in certain situations may not be obtained.

In view of this, the receiving signal channel switching sequencerelating to antenna transmission and reception in the radar apparatusshown in FIG. 1 is modified so that the multi-channel formation can beswitched between a coarse mode and a fine mode depending on theenvironment in which the radar is used for operation, and so that themode can be switched from the fine mode to the coarse mode when a fasterazimuth detection speed is needed. FIGS. 8 and 9 show a first embodimentof the antenna switching operation in the radar apparatus of the presentinvention.

In FIG. 8, as in FIG. 4, the timing for switching between thetransmitting antennas A1 to A4 and the receiving antennas A1 to A4 isshown in time series in conjunction with the triangular wave cycles ofthe transmitted FM-CW wave. In FIG. 8 also, receiving antenna 1indicates each of the transmitting antennas A1 to A4 that was selectedfor transmission of the FM-CW wave and that, upon transmission, wasswitched to a receiving antennas for reception of the reflected wave,and antenna 2 indicates each of the receiving antennas A1 to A4 that wasswitched to function as a receiving antenna other than the receivingantenna 1.

FIG. 9 shows how the receiving channels are formed, that is, thereceiving signal channel switching sequence relating to antennatransmission and reception, in the first embodiment of the antennaswitching operation performed in accordance with the switching timingchart shown in FIG. 8. In FIG. 9, as in FIG. 5, the horizontal axisrepresents the time, and triangles indicate receiving antennas, whileshaded triangles indicate antennas used for both transmission andreception. The receiving channels for the respective antennas are shownby reference to the transmitting antenna for each period of thetriangular FM-CW wave.

Unlike the earlier described first and second specific examples, in thefirst embodiment shown in FIGS. 8 and 9, the eleven receiving signalchannels are not always formed, but the eleven channels to be formed aredivided between a group of odd-numbered channels and a group ofeven-numbered channels. As shown in FIG. 8, of the plurality ofcontiguous triangular waves in the FM-CW wave having a length necessaryto form the eleven receiving channels, the first half is allocated tothe formation of the odd-numbered receiving channels and the second halfto the formation of the even-numbered receiving channels in thereceiving signal channel switching sequence. First, of the elevenchannels, the formation of the odd-numbered receiving channels will bedescribed. The first section shown in FIG. 9 contains the first threetriangular wave cycles of the FM-CW wave shown in FIG. 8, and the secondto fourth periods each contain the subsequent one triangular wave cycle.In the first period, the antenna A1 is selected as the transmittingantenna, and the three cycles of triangular waves are transmitted fromthe antenna A1, while the antennas A2, A3, and A4 are each selected asthe receiving antenna 2 for each transmitting cycle. The antenna A2receives the signal on the seventh channel, the antenna A3 receives thesignal on the ninth channel, and the antenna A4 receives the signal onthe 11th channel.

Next, in the second period, the antenna A2 is selected as thetransmitting antenna, the antenna A1 is selected as the receivingantenna 2, and the antenna A1 receives the signal on the fifth channel.Then, in the third section, the antenna A3 is selected as thetransmitting antenna, the antenna A1 is selected as the receivingantenna 2, and the antenna A1 receives the signal on the third channel.Further, in the fourth period, the antenna A4 is selected as thetransmitting antenna, the antenna A1 is selected as the receivingantenna 2, and the antenna A1 receives the signal on the first channel.In FIG. 9, any antenna that is not selected as the antenna 2 and doesnot contribute to the formation of the odd-numbered receiving channelsis shown by a dashed triangle.

As described above, of the eleven channels, the six odd-numberedreceiving channels are formed with the six triangular wave cycles of theFM-CW wave. Next, a description will be given of how, of the elevenreceiving channels, the even-numbered channels are formed with the fourtriangular waves that follow the above six triangular waves. As shown inFIG. 8, the even-numbered receiving channels are formed with the fourtriangular wave cycles in the second half, and in FIG. 9, the selectiontiming for each antenna selected as the receiving antenna 2 is shown bya dashed triangle.

As can be seen from FIG. 9, when forming the even-numbered receivingchannels, the first period does not have any contribution but the secondand fourth periods contribute to the formation of the receivingchannels. The second section contains two cycles of triangular wavesthat follow the triangular waves used for the formation of theodd-numbered receiving channels, and the fourth period contains twocycles of triangular waves that follow the above two triangular wavecycles; with these four triangular wave cycles, the even-numberedreceiving channels are formed.

First, in the second period, the antenna A2 is selected as thetransmitting antenna, and two cycles of triangular waves are transmittedfrom the antenna A2, while the antennas A3 and A4 are each selected asthe receiving antenna 2 for each transmitting cycle. The antenna A2 asthe receiving antenna 1 receives the signal on the sixth channel, theantenna A3 as the receiving antenna 2 receives the signal on the eighthchannel, and the antenna A4 receives the signal on the 10th channel.Then, in the fourth period, the antenna A4 is selected as thetransmitting antenna, and two cycles of triangular waves are transmittedfrom the antenna A4, while the antennas A2 and A3 are each selected asthe receiving antenna 2 for each transmitting cycle. The antenna A2 asthe receiving antenna 2 receives the signal on the second channel, andthe antenna A3 as the receiving antenna 2 receives the signal on thefourth channel.

As described above, of the eleven channels, the five even-numberedreceiving channels are formed with the four triangular wave cycles ofthe FM-CW wave. Therefore, by performing the formation of the fiveeven-numbered receiving channels immediately following the formation ofthe odd-numbered receiving channels earlier described, eleven receivingsignal channels can be formed with ten triangular wave cycles.

As in the second specific example shown in FIGS. 6 and 7, not only theperformance improvement due to the formation of the multiple channels,but the speedup of signal processing can also be achieved. Here, theformation of the six odd-numbered receiving channels earlier describedmay be performed immediately following the formation of the fiveeven-numbered receiving channels described above; in that case also,eleven receiving signal channels can be formed with ten triangular wavecycles.

In applications where the radar apparatus is mounted, for example, on avehicle such as an automobile and is used to detect the azimuth to atarget object located ahead, accurate azimuth detection using all thechannels is required in such situations as when the vehicle is runningat a slow speed or on a congested road; on the other hand, in normaldriving situations, it is required to increase the processing speed forazimuth detection. To address such requirements, when accurate azimuthdetection is required, the receiving signal channel switching sequenceis switched to the fine antenna transmission/reception mode, andmultiple beams are formed using all the eleven channels by performingthe receiving channel formation in accordance with the receiving signalchannel switching sequence for the odd-numbered channels, immediatelyfollowed by the receiving channel formation in accordance with thereceiving signal channel switching sequence for the even-numberedchannels; in this way, the directivity of the radar apparatus can beenhanced and high resolution can be achieved.

On the other hand, in situations where accurate azimuth detection usingall the channels is not required but it is required to increase theprocessing speed for azimuth detection, the receiving signal channelswitching sequence is switched to the coarse mode which is not a highresolution mode, i.e., to the switching sequence for forming only theodd-numbered receiving channels, and multiple beams are formed usingonly six channels out of the eleven channels; in normal drivingsituations, this switching sequence is repeatedly performed to increasethe processing speed for azimuth detection in the radar apparatus. Inthis way, the radar apparatus can be set up to match the usingenvironment of the apparatus without changing the configuration of theapparatus but by just switching the receiving signal channel switchingsequence relating to antenna transmission and reception to theappropriate mode.

FIGS. 10 and 11 show a second embodiment in which the receiving signalchannel switching sequence relating to antenna transmission andreception in the radar apparatus shown in FIG. 1 is modified so as toenhance the accuracy of azimuth detection; in this embodiment, themultiple receiving signal channels to be formed for the detection of atarget object located ahead of the radar apparatus are divided into twogroups, the left group and the right group, and multiple beams areformed using the plurality of channels in the left half and theplurality of channels in the right half, respectively, so that theprocessing for azimuth detection can be performed twice.

In FIG. 10, as in FIG. 4, the timing for switching between thetransmitting antennas A1 to A4 and the receiving antennas A1 to A4 isshown in time series in conjunction with the triangular wave cycles ofthe transmitted FM-CW wave. In FIG. 10 also, receiving antenna 1indicates each of the transmitting antennas A1 to A4 that was selectedfor transmission of the FM-CW wave and that, upon transmission, wasswitched to a receiving antennas for reception of the reflected wave,and antenna 2 indicates each of the receiving antennas A1 to A4 that wasswitched to function as a receiving antenna other than the receivingantenna 1.

FIG. 11 shows how the receiving channels are formed, that is, thereceiving signal channel switching sequence relating to antennatransmission and reception, in the second embodiment of the antennaswitching operation performed in accordance with the switching timingchart shown in FIG. 10. In FIG. 11, as in FIG. 5, the horizontal axisrepresents the time, and triangles indicate receiving antennas, whileshaded triangles indicate antennas used for both transmission andreception. The receiving channels for the respective antennas are shownby reference to the transmitting antenna for each section of thetriangular FM-CW wave. In the example shown in FIG. 11, the left-halfchannel group is the group consisting of the sixth to 11th channels, andthe right-half channel group is the group consisting of the first tosixth channels.

Unlike the earlier described first and second specific examples, in thesecond embodiment shown in FIGS. 10 and 11, the eleven receiving signalchannels are not always formed, but the eleven channels to be formed aredivided into the left-half and right-half groups each consisting of aplurality of channels. As shown in FIG. 10, of the plurality ofcontiguous triangular waves in the FM-CW wave having a length necessaryto form the eleven receiving channels, the first half is allocated tothe formation of the receiving channels in the left-half group and thesecond half to the formation of the receiving channels in the right-halfgroup in the receiving signal channel switching sequence. Here, thesixth channel which is set as the reference receiving signal channel isincluded in both the left-half and right-half receiving channel groups.Further, to enhance the resolution of the azimuth detection performedusing the plurality of channels in the left half and the right half, anadditional channel or channels may be included in each of the left-halfand right-half receiving channel groups so that each group consists ofseven or more channels.

First, of the eleven channels, the formation of the receiving channelsin the left half will be described. The first to sixth periods shown inFIG. 9 each contain one triangular wave cycle of the FM-CW wave shown inFIG. 10, and the seventh and eighth periods each contain two triangularwave cycles of the FM-CW wave shown in FIG. 10, that is, a total ofeight periods are provided to form the eleven channels. In the firstperiod, the antenna A1 is selected as the transmitting antenna, and onecycle of triangular wave is transmitted from the antenna A1, while theantenna A1 is selected as the receiving antenna 1 and the antenna A4 asthe antenna 2. The antennas A1 and A4 receive the signals on the sixthand 11th channels, respectively.

Next, in the second period, the antenna A2 is selected as thetransmitting antenna, while the antenna A2 is selected as the receivingantenna 1 and the antenna A4 as the receiving antenna 2, to receive thesignals on the sixth and 10th channels, respectively. In the subsequentthird to sixth sections, the antennas A1 and A2 are alternately selectedas the transmitting antenna, while the antennas A1 and A2 arealternately selected as the receiving antenna 1, and the antenna A2 orA3 corresponding to one of the ninth to seventh channels is selected asthe receiving antenna 2. With this switching sequence, the referencechannel signal is received, and the signals on the ninth to seventhchannels are sequentially received.

In the left-half receiving signal channel switching sequence performedin accordance with the antenna selection procedure described above,multiple beams can be formed with the six receiving signal channelsconsisting of the sixth to 11th channels. The signal processing meansperforms processing for azimuth detection based on the multiple beamsformed with the left-half six channels.

Next, the formation of the receiving channels in the right half will bedescribed. In the sixth section, the antenna A2 is selected as thetransmitting antenna, and one cycle of triangular wave is transmittedfrom the antenna A2, while the antenna A2 is selected as the receivingantenna 1 and the antenna A1 as the antenna 2. The antennas A2 and A1receive the signals on the sixth and fifth channels, respectively.

In the seventh period, the antenna A3 is selected as the transmittingantenna, and two cycles of triangular waves are sequentially transmittedfrom the antenna A3, while the antenna A3 is selected as the receivingantenna 1 and the antennas A2 and A1 are sequentially selected as theantenna 2. The antennas A2 and A1 receive the signals on the fourth andthird channels, respectively.

Next, in the eighth period, the antenna A4 is selected as thetransmitting antenna, and two cycles of triangular waves aresequentially transmitted from the antenna A3, while the antenna A4 isselected as the receiving antenna 1, and the antennas A2 and A1 aresequentially selected as the antenna 2 and receive the signals on thesecond and first channels, respectively. In FIG. 11, any antenna that isnot selected as the antenna 2 and does not contribute to the formationof the receiving channels is shown by a dashed triangle.

In the right-half receiving signal channel switching sequence performedin accordance with the antenna selection procedure described above,multiple beams can be formed with the six receiving signal channelsconsisting of the first to sixth channels. The signal processing meansperforms processing for azimuth detection based on the multiple beamsformed with the left-half six channels.

As described above, of the eleven channels, six receiving signalchannels in each of the left-half and right-half groups are formed withfive triangular wave cycles of the FM-CW wave. Accordingly, the signalprocessing means can perform the processing for azimuth detection forthe left-half and right-half groups separately, based on the signalsreceived on the six channels in the respective groups.

By performing the azimuth detection once for every five triangular wavecycles of the FM-CW wave as described above, the processing speed forazimuth detection can be increased, as in the case of the receivingchannel formation for the odd-numbered channels in the first embodiment.Furthermore, as the processing for azimuth detection can be performedonce again by using the next five triangular wave cycles of the FM-CWwave, which means repeating the azimuth detection of the same resolutiontwice, the accuracy of the azimuth detection can be enhanced.

In the radar apparatus shown in FIG. 1, the four antennas aresequentially switched by the switching means 5 into operation as thetransmitting antenna, starting from one end of the antenna array, but ifthe antennas are sequentially selected starting from an antenna locatedhalfway along the antenna array, not from one end of the antenna array,the eleven channels can be formed in the same manner as when theantennas are sequentially selected starting from one end of the array.

In the examples so far described, all of the four antennas have beensequentially selected as the transmitting antenna to achieve the elevenchannels, but there are situations where such high beam directivity isnot needed. For example, consider the case where the radar apparatus ismounted on a vehicle; in this case, if the vehicle speed is so fast thatthe computing speed of the radar apparatus cannot keep up with thevehicle speed, and if the distance to the target object is closing, itmay become necessary to reduce the amount of computation and increasethe computing speed.

To address such situations, only five channels may be obtained bysequentially selecting, for example, the antennas A1 and A2 as thetransmitting antenna rather than sequentially selecting all of the fourantennas as the transmitting antenna in the radar apparatus, oralternatively, nine channels may be obtained by sequentially selectingthe antennas A1, A2, and A3 as the transmitting antenna. In this way, bychoosing an appropriate set of antennas for selection as thetransmission antenna, the number of channels to be formed can be changedwithout changing the four-antenna configuration but by just controllingthe switching operation of the switch SW.

The embodiments of the radar apparatus described above have dealt withthe case where the apparatus has four transmit/receive common antennas,but in order to obtain a suitable number of channels, all the fourantennas need not necessarily be used for both transmission andreception, but two or three of the four antennas may be used for bothtransmission and reception. On the other hand, if it is desired tosignificantly increase the number of channels, an antenna A5 (not shown)may be added at a position spaced apart by 2d.

Further, in each of the embodiments of the radar apparatus describedabove, the apparatus has four transmit/receive common antennas, anddigital beams of eleven channels are formed using a space equivalent tosix antennas. Here, if the number of antennas is reduced to three, theobject of the invention to form digital beams having a larger number ofchannels with a fewer number of antennas can be achieved. When thenumber of antennas is three, the maximum number of channels that can begenerated is seven. In this case, the apparatus size can be made smallerthan the prior art DBF radar with nine channels, though the resolutiondrops.

On the other hand, if the number of antennas is increased to five ormore, the resolution increases, but it becomes correspondingly difficultto achieve size and cost reductions. Accordingly, when the number ofantennas is four, digital beams can be formed with a maximum of elevenchannels that can be generated, and not only does it become possible toachieve a resolution higher than that of the antennas in the prior artDBF radar apparatus, but the apparatus size can be reduced while alsoreducing the cost of the apparatus.

It can therefore be said that the four-antenna configuration is the bestchoice for the formation of multi-channel digital beams. In this way,when four antennas are provided, the number of channels to be generatedcan be reduced to form digital beams with fewer than eleven channels inorder, for example, to address situations where the speed of azimuthdetection is more critical than the detection accuracy. For example, thenumber of channels can be reduced to nine to reduce the resolution tothe level comparable to that of the prior art DBF radar apparatus.Accordingly, the four antennas of the radar apparatus of the presentinvention preserve compatibility with the antennas of the prior art DBFradar apparatus.

As described above, in the configuration of the radar apparatus shown inFIG. 1, of the plurality of antennas arranged in a straight line on thesame plane, the radiowave is transmitted from at least one selectedantenna, and the reflected waves of the transmitted radiowave arereceived by the respective antennas; accordingly, not only can a largernumber of channels be obtained with fewer antennas than that inconventional radar apparatuses, but also the size and cost of the radarapparatus can be reduced. In particular, when all of the plurality ofantennas are used for both transmission and reception, the number ofchannels can be greatly increased, and the directivity when the receivedsignals are combined can be increased, serving to enhance theperformance of the radar apparatus.

Next, modified examples of the above embodiments will be described withreference to FIGS. 12 to 18; the modified examples shown below are basedon the configuration of the radar apparatus of the present inventionshown in FIG. 1, and concern the configurations for improving theefficiency, reducing the cost, or achieving further reduction in size inaccordance with the receiving signal channel switching sequences of thefirst and second embodiments.

In a first modified example shown in FIG. 12, the switching means 5 isomitted, and transceivers 61, 62, 63, and 64, each serving as both thetransmitter 2 and the receiver 4, are respectively connected to the fourantennas A1, A2, A3, and A4. This configuration is effective inapplications where it is required to prevent signal attenuation even ifthe cost increases. A voltage-controlled oscillator (VCO) for generatingthe transmit signal may be provided at each transceiver, but in theillustrated example, a single voltage-controlled oscillator 3 isprovided common to all the transceivers to share the same transmitsignal source among them and to facilitate the synchronization ofreceiving operations, while achieving a reduction in cost. A duplexermeans for switching each antenna between transmission and reception canbe constructed, for example, from a hybrid circuit or a distributioncircuit; further, an amplifier (AMP) and an attenuator (ATT) may beinserted at the transmitting and receiving sides so that thetransmission and reception can be turned on and off by controlling thegain or the amount of attenuation. In this way, each antenna can beswitched between transmission and reception in time division fashion.

A second modified example shown in FIG. 13 employs a configurationintermediate between the configuration shown in FIG. 1 and theconfiguration shown in FIG. 12, and aims at reducing the signalattenuation to a relatively low level while also reducing the costincrease due to the addition of the transceivers. To this end, ratherthan omitting the switching means 5 as in the example of FIG. 12, theconfiguration of FIG. 13 employs single-pole double-throw (SPDT)switches each for switching between two antennas, and the switches SW1and SW2 are controlled by the signal processing control unit 1 to switchbetween the four antennas. In this case also, the voltage-controlledoscillator is provided common to the transceivers 65 and 66.

The above has described the modified examples of the embodimentsconcerning the configuration relating to the transmitter and receiver;now, specific examples of the switching means 5 in the radar apparatusof FIG. 1 will be described below. The figures given hereinafter focuson the configuration of the switching means 5, and therefore, the signalprocessing control unit 1 and the voltage-controlled oscillator 3 whichare essential to the radar apparatus are not shown. The four antennasA1, A2, A3 and A4 forming the antenna array A, the transmitter 2, andthe receiver 4 are connected to the switching means 5, and the switchingoperation of the switch SW is controlled by the signal processingcontrol unit 1 so that the four antennas are switched betweentransmission and reception in time division fashion.

In a first specific example shown in FIG. 14, each of the four antennasA1, A2, A3 and A4 is provided with a transmitting port and a receivingport so that the antennas can be connected to the transmitter 2 or thereceiver 4 in time division fashion.

In a second specific example shown in FIG. 15, contrary to the firstspecific example, the transmitting and receiving ports are provided oneach of the transmitter 2 and receiver 4 sides. In this example also,the antennas can be connected to the transmitter 2 or the receiver 4 intime division fashion.

In a third specific example shown in FIG. 16, each of the four antennasA1, A2, A3 and A4 is provided with one port so that each antenna can beselected for operation from the transmitter 2 or the receiver 4 side. Inthis example also, the antennas can be connected to the transmitter 2 orthe receiver 4 in a time-division fashion.

While, in the third specific example, the transmitter 2 and the receiver4 can independently control each antenna for connection, in a fourthspecific example shown in FIG. 17 a duplexer means is provided withinthe switching means 5 so that the antennas can be connected to thetransmitter 2 or the receiver 4 in a time-division fashion. The duplexermeans can be constructed from a hybrid circuit or a distribution circuitto achieve an inexpensive circuit configuration.

In a fifth specific example shown in FIG. 18, the duplexer means in thefourth specific example is constructed from a single-pole double-throwswitch; in this example also, the antennas can be connected to thetransmitter 2 or the receiver 4 in time division fashion.

Here, in the first to fifth specific examples, if the switch SW in theswitching means 5 is constructed from a bidirectional switch that can beused for both transmission and reception, the size of the switchingmeans 5 can be reduced.

1. A radar apparatus comprising: a plurality of antennas havingidentical antenna characteristics, being arranged at unequally spacedintervals in a single row, the plurality of antennas configured totransmit a radio wave as transmitting antennas during continued multipleperiods, and receive a reflected radio wave as receiving antennas duringthe continued multiple periods, said receiving antennas being assignedto receiving signal channels, said receiving signal channels beingnumbered from 1 to N, said N being larger than the number of antennas; aplurality of transceivers for transmitting a radio wave from thetransmitting antennas and receiving received signals from the receivingantennas, said received signals representing the reflected wave receivedat the receiving antennas; and a signal processing unit for selecting atransmitting transceiver which transmits a radio wave during each of thecontinue multiple periods, selecting receiving transceivers whichreceive the received signals from the receiving antennas assigned to thereceiving signal channels during each of the continued multiple periods,and performing digital beam forming with a first receiving signalchannel group and a second receiving signal channel group respectively.2. The radar apparatus claimed in claim 1, wherein the first receivingsignal channel group comprises odd-numbered receiving signal channelsand the second receiving signal channel group comprises even-numberedreceiving signal channels.
 3. The radar apparatus as claimed in claim 1,wherein the first receiving signal channel group comprises a first halfof the N channels and a second receiving signal channel group comprisesthe second half of the N channels.
 4. The radar apparatus as claimed inclaim 1, wherein all of the plurality of antennas are used for bothtransmission and reception.
 5. The radar apparatus as claimed in claim1, wherein receiving antennas are assigned to the receiving signalchannels during each of the continued multiple periods with reference tothe transmitting antenna of the same period.
 6. The radar apparatus asclaimed in claim 1, wherein the signal processing unit (1) adjustsphases of the received signals during each of the continued multipleperiods based on a phase of the received signal received from thereceiving antenna that is the same antenna to the transmitting antennaof the same period, and (2) performs digital beam forming based on thephase adjusted received signals.
 7. The radar apparatus as claimed inclaim 1, wherein in an arrangement of the plurality of antennas, a ratioof antenna spacing between a predetermined pair of adjacent antennas toantenna spacing between another pair of adjacent antennas is 1:2.
 8. Theradar apparatus as claimed in claim 7, wherein the plurality of antennasinclude a first, second, third, and fourth antennas arrayed in sequencealong a straight line, the first and second antennas are arranged with afirst spacing, and the second and third antennas and the third andfourth antennas are arranged with a second spacing, and wherein thesecond spacing is twice as large as the first spacing.
 9. The radarapparatus as claimed in claim 8, wherein the signal processing unitforms multiple digital beams of eleven channels based on the receivedsignals received at the first, second, third, and fourth antennas duringthe continued multiple periods.
 10. The radar apparatus as claimed inclaim 1, further comprising a selector switch for sequentiallyconnecting one of the transceivers to at least two of the antennas,wherein the number of transceivers is smaller than the number ofantennas.
 11. The radar apparatus as claimed in claim 10, wherein thetransceiver is switched for transmission or reception in time divisionfashion.
 12. The radar apparatus as claimed in claim 1, wherein thefirst receiving signal channel group comprises all of the N channels andthe second receiving signal channel group comprises odd-numberedreceiving channels or even-numbered receiving channels, and the signalprocessing unit performs azimuth detection with either one of thereceiving signal channel group in accordance with an environment. 13.The radar apparatus as claimed in claim 1, wherein the first receivingsignal channel group comprises all of the N channels and the secondreceiving signal channel group comprises the first half of the Nchannels or the second half of the N channels, and the signal processingunit performs azimuth detection with either one of the receiving signalchannel groups in accordance with an environment.
 14. The radarapparatus as claimed in claim 1, wherein the plurality of antennasinclude four antennas arranged at unequally spaced intervals along astraight line, and the signal processing unit forms multiple digitalbeams of eleven channels based on the received signals received at therespective antennas during the continued multiple periods.
 15. The radarapparatus as claimed in claim 1, wherein the plurality of antennasinclude four antennas arranged at unequally spaced intervals along astraight line, and the signal processing unit forms multiple digitalbeams of a plurality of channels fewer than eleven channels based onsome of the received signals received at the respective antennas duringthe continued multiple periods.